Teachers unions have a well-deserved reputation for exercising political clout. With a nearly unparalleled ability to raise cash and organize their ranks, they have elected school boards, influenced legislation and helped set the public school agenda in major American cities for decades.

Now, that clout is in question.

A nationwide school reform movement with bipartisan support has collided head-on with unions over three ideas that labor has long resisted: expansion of charter schools, the introduction of merit pay for teachers and the use of student test scores in teacher evaluations.

Even the long-held protections and prerogatives conferred by seniority and tenure no longer seem sacrosanct.

"To say that we're under attack is an understatement," Los Angeles teachers union vice president Julie Washington told an angry audience of her members recently. "This is a wake-up call for all of us."

It's not that unions have been slumbering, but they have been slow to come to terms with the surging momentum for change. Critics see them as obstacles; even union sympathizers agree that their voice in the education debate has been muted.

"The big ideas that are being debated are not the ideas that they put there," said Charles Kerchner, a professor of education at Claremont Graduate University, who has written several books about teachers unions. "They're not forming the agenda."

Or as Jay Greene, a New York-based education researcher and union critic, recently blogged: "We won! At least we've won the war of ideas."

Unions' headaches begin at the top, with President Obama and his education secretary, Arne Duncan, Democrats who have pursued an agenda that builds substantially on the policies of Republican President George W. Bush.

Teachers unions donate almost exclusively to Democratic politicians and have usually been able to count on their support. Obama has disappointed them -- and the feeling appears to be mutual.

Asked recently whether teachers unions were getting in the way of progress, the president said: "I'm a strong supporter of the notion that a union can protect its members and help be part of the solution as opposed to part of the problem. What is also true is that sometimes means they are resistant to change when things are not working."

Locally, opposition from a strong union, United Teachers Los Angeles, hasn't been enough to stop the creation of more charter schools than in any other city in the country. These schools -- independently operated and publicly financed -- are sometimes unionized, but most are not.

Across the country, dozens of states and school districts have proposed or instituted changes in the way they evaluate teachers to take into account how much their students improve on standardized tests. In Los Angeles, some district officials are pushing to rate individual teachers in this way -- over strenuous union objections.

The pressure has grown since August, when The Times published a database that rated about 6,000 elementary school teachers using the "value-added" method. A series of articles underscored significant differences in teachers' influence on test scores, even within the same schools. Union leaders unsuccessfully urged the newspaper to take down the database, saying it was unfair and based on flawed results. The union has called for a boycott of the newspaper and alleged, among other things, that a teacher killed himself in response to a "less effective" rating in the database.

In October, UTLA expressed outrage at the district over a proposed legal settlement with civil rights attorneys that could threaten a longstanding "last hired, first fired" principle. The union had been part of the negotiations but ended up on the sidelines.

In New York City, the United Federation of Teachers was left fuming too, as school officials there announced plans to release teacher evaluation data to the public despite an earlier promise that they would try to keep the information private.

These developments hardly mean the unions have become impotent.

"They're still plenty powerful," Greene said. "They still have millions of dollars and millions and millions of members."

Union supporters say organized labor has not been given enough credit for its own efforts to reform education -- for instance, efforts by UTLA to establish charter-like "pilot schools." And, they insist, there is scant evidence that the education reforms championed by Obama, Duncan and others will do anything to improve schools.

"I think a lot of the politicians ... are looking for a quick fix, and they're looking for a scapegoat, and the scapegoat in this case is the teachers," said Kirti Baranwal, a teacher and union representative at Gompers Middle School in South Los Angeles.

Not so many years ago, teachers unions were popular and powerful advocates for education reform in major U.S. cities, pushing through winning changes in Los Angeles, New York and other districts that gave teachers significant power over what and how they taught and gave schools leeway to manage themselves.

In many ways, their sway is still felt: They continue to spend vast sums to support candidates and issues at the federal, state and local levels.

In 2008, the single largest contributor to state and federal campaigns was the National Education Assn., which spent $56 million, according to the nonpartisan Center for Responsive Politics. The California Teachers Assn. has spent $211 million in the last decade trying to influence state campaigns, roughly double the amount of the next largest group, according to a report by the state Fair Political Practices Commission.

In local school board campaigns, not only are unions usually the largest spender by far, they typically supply the largest volunteer force of campaign workers.

"They have activists ringing doorbells and making phone calls," said Terry Moe, a professor at Stanford University whose work focuses on teachers unions. "They are awesome. You don't want to get on the wrong side of the teachers unions if you're a politician."

And yet, as the national focus has shifted toward holding teachers and schools accountable for students' academic achievement, even long-time supporters have been willing to buck the unions and risk the consequences.

State Sen. Gloria Romero (D-Los Angeles) enjoyed strong union support -- until she became an outspoken advocate for charter schools, value-added assessments and other changes.

When she ran for the nomination for state superintendent of public instruction this year, the CTA shunned her and heavily supported one of her opponents, Tom Torlakson, who won the post last week. Although Romero may have been the best-known figure heading into the June primary, she finished third.

She isn't alone among labor-friendly politicians in breaking ranks -- at least once in a while.

On the day Steve Zimmer was sworn in as a member of the L.A. Board of Education, the leadership of UTLA had reason to be satisfied, if not smug.

As a teacher at Marshall High School, Zimmer had been an active member of the union, which represents 40,000 teachers. The union shelled out more than $350,000 for his campaign. In his first speech on the board, Zimmer spoke movingly of a commitment to trade unionism rooted in his great-grandparents' battles to organize Brooklyn garment workers.

Within minutes of that speech July 1, 2009, the board took up a proposal that would turn over some campuses to outside entities, including charter operators -- anathema to the union.

Zimmer sided with the majority in favor of the plan and against the leadership of the union.

"That vote," Zimmer said recently, "cost me lifelong friendships."

Zimmer has rarely differed with the union since then. Still, he represents in some ways the new complexity facing teachers unions. He is an alumnus of Teach For America, which places college graduates in low-performing schools.

They are mostly a smart, liberal group, many of whom have been critical of teachers unions. (Indeed, perhaps their most famous alumna, Michelle Rhee, became the bete noire for unions nationally before she resigned last month as chancellor of schools in Washington, D.C.)

The problems of unions extend into the culture at large. Consider Davis Guggenheim, the director of "Waiting for 'Superman,' " a documentary that paints unions as enemies of reform. He said that, as a Democrat, he believes in "the essence" of unions but that they can be on the right side -- or the wrong side -- of change. Even Oprah Winfrey has joined the fray, demanding to know, "Why can't you just fire bad teachers?"

A.J. Duffy, president of UTLA, said he believes that the challenges unions are facing are cyclical, not permanent, and that they are motivated by a pernicious corporate influence in education. "I think there's a fear that public education will be dismantled, rather than fixed."

Duffy said he sees his union as a driver of reform, but is skeptical -- some would say recalcitrant -- about the sort of measures favored by Duncan and others.

On teacher evaluations, he said, "if this breaks down to 'We're going to change the evaluation system just so we can have a tool to get rid of what we consider to be bad teachers,' then we will continue to fight. But if the district and the community wants to have a dialogue about what a meaningful evaluation system is, along with all the components that go into it, then we're there. We want it."

More perhaps than other union leaders, Randi Weingarten, president of the American Federation of Teachers, has taken the stance that it's better to help shape changes than to be left out of the discussion. She has touted her support of contracts that embody some new reform elements, including value-added assessments.

"No teacher -- myself included -- wants a bad teacher in any classroom," Weingarten said in a speech to union delegates in July. She said teachers need to "lead and propose, not wait and oppose."

Adam Urbanski, a prominent advocate for union-backed change as president of the Rochester, N.Y., teachers union and director of the Teachers Union Reform Network, said he thinks all sides in the debate over public education need to be more flexible if schools are to get better.

"I think it'll be tough enough if we all pull together," he said, referring to unions, districts and everyone else.